firstly, you should select a online optical store which can assure the quality and can give you full refund in the case the glasses are not fit for you! what's more, if you want to buy prescription glasses, it is a must to provide the online optical vendor with a copy of your prescription! at last, i suggest you to go to the local store to try on the glasses you want and then go to online optical store to purchase the cheaper one!
Buying eyeglasses online is the same way as you buying other items from online. First of all, you shall decided which shop to buy eyeglasses. Then choose eyeglasses frame at their sites and continue the order by input your prescription online. Next, you just go on the order step by step as they remind you. I believe you will do it successfully. Good luck.